Over the years, because Sir Arthur Conan Doyle created the character of Sherlock
Holmes who solved various mysteries, many people wrote letters to Conan Doyle
asking him for help with real life crimes. Conan Doyle was also a member of an
organisation called the Crimes Club which began in 1904 and which met regularly
to debate famous criminal cases, and included other writers of his day.
Evidence from real unsolved crimes was available for all members to study,
and lectures on methods of crime were given by actual criminals.

from: The Times, Wednesday, Aug 21, 1912; pg. 5

Conan Doyle also wrote for a series in the Strand Magazine
on the history of crime in 1901, called Strange Studies
from Life. Sometimes, however, he did get directly involved
with criminal cases, themselves. The two most famous examples
were George Edalji and Oliver Slater. Conan Doyle's involvement
in both these cases led to the establishment of the Court
of Criminal Appeal in both England and Scotland. The BBC created
a documentary on both these cases entitled
Conan Doyle for the Defence. (See the BBC site).

In 1903, over a period of six months in Great Wyrley, near
Birmingham, 16 animals were found slashed to death with shallow
slits along their stomach. George Edalji was convicted specifically
for an injured pony mutilated in the middle of the night and
discovered in a field not far from Edalji's home. His trial
inferred that he was also responsible for the other slayings.
Staffordshire police charged him of mutilating animals and
writing a threatening letters; some strangely enough accused
Edalji himself of the mutilations. That the animal slayings
continued after George Edalji’s imprisonment was discounted
by the local authorities, who by now had come up with a theory
that a cult was continuing the work Edalji had started. Conan
Doyle believed this was a case of blatent racism both on the
part of the local authorities and other unknown culprits,
due to Edalji's mixed heritage--his father was Indian and
his mother English.

In 1908 a wealthy 82-year-old woman named Marion Gilchrist, was bludgeoned to death at her home in Glasgow. Her body was discovered by a downstairs neighbour and a maid.
Oscar Slater, a petty criminal known to police as an illegal gambling-den operator had been staying in Glasgow for only 6 weeks, near the Gilchrist residence, before he boarded an ocean liner
for New York. When he found out he had been accused of Gilchrist's murder Slater returned to Glasgow to clear his name. However, his trial, characterized by
Conan Doyle as a great injustice, left Slater imprisoned for nearly two decades on sketchy evidence.
Conan Doyle believed the actual murderer was a member of the victim's family and known to the police. However because of that person's political and
social connections a cover-up was necessary and Slater became the perfect scapegoat for the crime. Read more about this case.

-Rose Roberto

Writings on these topics by Conan Doyle are in:

Strange Studies from Life, Strand Magazine (1901) republished as
Strange Studies from Life and Other Narratives: The Complete True Crime Writings
of Sir Arthur Conan Doyle (1963)
The Case of Mr. George Edalji (1907)
The Crime of the Congo (1909)
The Case of Oscar Slater (1912)
Letters to the Press (compiled and republished 1986)